In a typical radio communications network, communication devices, also known as mobile stations and/or User Equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be downlink and/or uplink cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
FIG. 1 illustrates a schematic example of the E-UTRAN comprising two eNBs, eNB1 and eNB2, and two Mobility Management Entities (MME), MME1 and MME2, in the Core Network (CN). The MME is used as a control node. For example, the MME is responsible for idle mode UE tracking and paging procedure including retransmissions. The MME is further involved in the bearer activation/deactivation process and is also responsible for choosing the serving gateway (SGW) for a UE at the initial attach and at time of intra-LTE handover involving CN node relocation. The MME is further responsible for authenticating the UE or user of the UE.
The two MMEs in FIG. 1 belong to an MME pool 1. An MME pool is a group of MMEs all connected to all eNBs in an MME pool area. The mobiles connected to each eNB may be served by different MMEs in the pool, to achieve load balancing between the MMEs and redundancy in case of MME unavailability.
Handover is an important aspect of any wireless communications network. In the text and some of the figures comprised herein handover is abbreviated with HO. With the handover the system tries to assure service continuity of the UE by transferring the connection between the communications network and the UE from one cell to another cell. When and to what cell the handover occurs depends on several factors such as signal strength of reference signals, load conditions in the cells, service requirements of the UE, etc. The provision of efficient and effective handovers, e.g. quantified by minimum number of unnecessary handovers, minimum number of handover failures, minimum handover delay, may affect not only the QoS of the end user such as the UE but also the overall mobile network capacity and performance.
In LTE, handover controlled by the communications network and assisted by the UE is utilized, for example described by 3GPP TS 36.300. The handover is based on UE reports. The UE is moved, if required and if possible, to the most appropriate cell that will assure service continuity and quality.
Handover is performed via a connection over an X2 interface between the eNB1 and the eNB2, whenever available. If the X2 is not available, handover is performed using a connection over an S1 interface between the eNB and the MME, i.e. involving the CN. The interfaces illustrated with full lines indicate that there is a functioning connection. The S1 interface between eNB2 and MME1 illustrated with a dashed-dotted line indicates that eNB2 is not S1-connected to MME1.
The handover procedure may be sub divided into three stages of preparation, also referred to as initiation, execution and completion.
The MME that serves the UE via the source eNB must also be connected to the target eNB as a prerequisite for X2 HO. The source eNB checks if this is the case by comparing what MME pools the currently serving MME belongs to with what MME pools the target eNB is connected to. In case the target eNB is connected to any of the MME pools to which the MME serving the UE belongs, the X2 HO may be chosen, otherwise S1 HO.
FIG. 1 shows an example of an inhomogeneous MME pool configuration. In this inhomogeneous MME pool configuration eNB2 is only S1 connected to a subset of the MMEs in the MME pool.
An inhomogeneous MME pool configuration may occur in the following scenarios:                Temporary S1-MME link failures        Faulty configuration of the MME pools        Transition phase when adding new MME to an existing MME pool. Not all eNBs may have established S1 connection to the new MME simultaneously.        
For inhomogeneous MME pool configurations X2 HO may not be possible even though the target eNB is connected to an MME pool to which the MME serving the UE belongs.
Taking FIG. 1 as an example the source eNB1 may know that target eNB2 is connected to the MME pool to which MME1 belongs. MM1 is serving the UE to be handed-over and eNB1 selects X2 HO as the handover type. However at this time the S1 link between eNB2 and MME1 suffers from link failure which causes the X2 HO to fail with a cause set to unknown MME-code. Note that eNB2 is still connected to MME pool 1, since there is an S1-MME connection to MME2.